The invention described herein was made in the course of, or under a contract with the United States Department of Energy. The invention relates generally to acid digestion processes and more particularly to the chemical digestion of combustible, low level radioactive, solid waste material.
Disposal of radioactive waste is an important problem in the nuclear energy field today since many radioactive wastes must be stored for very long time periods to assure that no health hazard will be incurred. Low level radioactive, combustible, solid waste materials are a particular problem because of the relatively large bulk of such materials associated with small amounts of contamination. Typical combustible, solid waste materials of concern are those resulting from fuel fabrication operations, such as used rubber gloves, paper, rags, brushes and various plastics. Of particular concern as well is the disposal of spent ion exchange resins from reactors, fuel fabrication plants and reprocessing plants (e.g. estimated to comprise from 500 to 800 cubic feet of material per year per nuclear reactor).
Present practice consists of packaging the solid waste materials in containers ranging from cardboard boxes lined with plastic bags to steel drums and then burying the packages in pits or trenches. This technique involves transporting the packaged materials over roadways and finally storing the materials in monitored repositories. Potential release of contamination to the environment is possible as a result of decay of the containers, or inadvertent combustion, etc. Moreover in fuel reprocessing plants and fuel preparation plants, spent ion exchange resins contain significant amounts of plutonium as well as other fission products, which may preclude direct burial of these resins.
Inasmuch as a large percentage of the contaminated solid waste material is simply light-weight, bulky, combustible material, incineration of solid nuclear waste materials has been studied extensively, but it is subject to poor control of combustion, with attendant off-gas system difficulties and severe corrosion problems, coupled with expensive maintenance problems. Mechanical compaction of the solid waste material has also been studied extensively with volume reductions of two- to five-fold being achieved. In general, however, compaction and sorting of solid waste materials are moderately expensive in that special personnel protection devices are needed over and above normal protective equipment costs and these operations do not put the material into an inert form.
In another approach a process based on the use of sulfuric acid with a selenium catalyst has been used to reduce the volume of combustible, low level radioactive waste. This process is described in "Treatment of Combustible, Solid, Low Level Radioactive Waste at RISQ, the Danish Atomic Energy Commission Research Establishment", Proceedings of a Symposium on Practices in the Treatment of Low and Intermediate Level Radioactive Waste, IAEA and ENEA, Vienna, December, 1965. While this process affords volume reductions approaching 60, the process requires the use of a very toxic catalyst and apparently has poor control of the reaction rate.
An improved system for the digestion of low level radioactive solid waste material has been described in U.S. Pat. No. 3,957,676, issued May 18, 1976. In the patented process the waste material is digested by reacting the combustible, solid waste with concentrated sulfuric acid at a temperature within the range of 230.degree. to 300.degree. C. and simultaneously and/or thereafter contacting the reacted mixture with concentrated nitric acid or nitrogen dioxide. The process is conducted batchwise or by incremental additions of solid waste materials and nitric acid or nitrogen dioxide. While a significant improvement in volume reduction in the order of up to 160 can be achieved with very little acid consumed, the waste through-put rate is relatively low, because of the geometrical limitations that must be imposed when treating fissile containing materials and this detracts from its practical value.
It is therefore desirable, and a primary object of this invention, to provide an improved system to that described in U.S. Pat. No. 3,957,676, that has a controlled, safe, less expensive and more readily manageable form of treatment of low level radioactive, combustible, scrap material, with suitable volume reductions and a relatively high through-put.